Mass filter electrode

ABSTRACT

A mass filter having glass electrodes which are at least partially coated with a continuous vapor deposited or sputtered thin metallic film of predetermined thickness or thicknesses. The electrodes are arranged about a central axis and receive electrical potentials that create a time periodical electric field therebetween to separate ions having different mass-tocharge ratios.

United States Patent McGinnis [54] MASS FILTER ELECTRODE [72] Inventor: Patrick F. McGinnis, Pittsford, NY. [73] Assignee: The Bendix Corporation [22] Filed: Feb. 22, 1971 [21] Appl. No.: 117,599

[52] US. Cl ..250/41Q9 DS, 161/196, 313/78 [51] Int. Cl ..H01j 39/34 [58] Field of Search ..250/41.9 DS; 313/78; 315/3;

[56] References Cited UNITED STATES PATENTS 3,350,559 10/1967 Young et a1. ..250/41.9 2,945,124 7/1960 Hall et a1. ..250/4l.9 2,939,952 6/1960 Paul et a1 ..250/41.9 3,544,832 12/1970 Brous ..117/212 X that create a time [451 Oct. 17,1972

OTHER PUBLICATIONS A Simple Quadrupole Mass Spectrometer by A. R. Fairbairn from The Review of Scientific Instruments, Vol. 40, No. 2, Feb., 1969, pgs. 380 and 381.

Primary Examiner-William F. Lindquist Attorney-Raymond J Eifier and Flame, l-lartz, Smith and Thompson [57] ABSTRACT A mass filter having glass electrodes which are at least partially coated with a continuous vapor deposited or sputtered thin metallic film of predetermined thickness or thicknesses. The electrodes are arranged about a central axis and receive electrical potentials periodical electric field therebetween to separate ions having different massto-charge ratios.

16 Claims, 7 Drawing Figures marrow 11 m2 SHEET 1 BF 2 FIGURE l PRIOR ART DC SOURCE RECTIFIER AC GEN ERATOR FIGURE 2 PRIOR ART PATRICK E Mcemms INVENTOR.

ATTORNEY P'A'TENTEBucI I 7 I972 3.699.330

SHEET 2 or 2 FIGURE 3 FIGuRE 4 FIGURE 5 DIRECT CURRENT H| GH FREQ. VOLT SUPPLY GENERATOR SOURCE AMPERES RECORD L Nil L FIGuRE 7 4 I 4 E I 5 Z l 6 42 PATRICK F MG|NNIS FIGURE 6 INVENTOR- BACKGROUND OF THE INVENTION This invention relates to a mass filter of the type having a plurality of axially elongated electrodes arranged about a central axis for creating a time periodical electric field therebetween to separate ions having different mass-to-charge ratios. The invention is more particularly related to the elongated electrodes and the thickness of a conductive thin film thereon which is utilized to shape the electric field between the electrodes, especially at the input and output ends of the filter.

In the operation of a nonmagnetic mass analyzer, such as a quadrupole mass filter of the type described in U.S. Pat. No. 2,939,952 to W. Paul et al., it has been found that exposure of charged particles to be analyzed to the electric fields existing at'the entrance end of the analyzer (fringing field) for more than two or three cycles of the ac. voltage applied to the analyzer field forming electrodes results in an undesirable radial impulse being imparted to the particles entering the filter. If the impulse is sufficiently high, the charged particles, that are to be analyzed, contact the electrodes where they are discharged, thereby reducing the quantity of charged particles which would otherwise be subject to analysis. Accordingly, the sensitivity of the mass filter is reduced, which is undesirable. Similarly, the fringing field at the exit end of the mass analyzer is also a problem.

A first approach to reducing the undesirable impulse producing effects of the fringing field has been to impart a relatively high injection energy to particles as they are directed towards the entrance end of the filter so as to reduce the transient time of the particles through the fringing fields to a minimum. A high injection energy, however, limits the maximum resolving power which can be obtained and imposes additional electric power requirements on the instrument to restore some of the lost resolving power, an analyzer of increased length is often provided in order to permit the analyzing or resolving action of the electric field within the analyzer to be exerted over a longer distance and particle transient time. Increased analyzer lengthis frequently undesirable especially when the analyzer is made part of the instrumentation in a space vehicle.

A second approach has been to provide auxiliary electrodes adjacent to the entrance end of the instrument. The ratio of the voltages connected to these electrodes is then arranged such that particles entering the analyzer encounter an intermediate ratio in the transition from the region of zero field outside the analyzer to the region of very strong electric fields in the center of the analyzer. This is done by reducing the amplitude of the dc. voltage connected to the auxiliary electrodes while leaving the amplitude of the a.c. voltage unchanged. By this means, a substantial increase in the transmission efficiency of the analyzer is achieved. Such an approach is described in U.S. Pat. No. 3,129,327 to W. M. Brubaker.

. A third approach has been to try to shape the electric field of the mass filter by having a plurality of separate and distinct electric fields along the axis of the filter. This was achieved by having each filter electrode con- 'sist of a plurality of segments each of which are electrically isolated from the other so that different voltage sensitivity of a quadrupole mass filter.

potentials could be applied thereto. This increased the sensitivity of the filter by allowing more particles to pass through the unstable fringing areas without being lost. However, this was a complex and expensive means and resulted in noncontinuous electrical fields at the surface of the electrodes. Further, since each segment was electrically isolated from the other, individual electrical connections to a voltage potential were required. Examples of this type of approach can be found in U.S.

Pat. No. 3,371,204 to W. M. Brubaker and No. 3,147,445 to R. F. Wuerker et al.

SUMMARY OF THE INVENTION To reduce the undesirable effect of the electric field on ions entering a nonmagnetic mass filter and thereby increase the sensitivity of the filter, especially a quadrupole type filter, the electrodes of the filter are comprised of glass rods having a thin conductive film thereon which may have a plurality of thicknesses.

The invention is characterized by a mass filter having one or more of the electric field forming electrodes comprised of glass having a conductive film thereon which is of a predetermined thickness or thickness extending a-predetermined length along the surface of the glass electrodes. In one embodiment of the invention, each electrode of a mass filter is comprised of a glass rod having a predetermined resistive surface and a metal film deposited thereon by vapor deposition or sputter coating techniques to achieve a desired thickness and thereby a desired resistance ratio between the resistive surface of the glass rod and the metal film.

Accordingly, it is an object of this invention to have a nonsegmented electrode that has a conductive film thereon that changes the electrical characteristics of the surface of the electrode and theelectric field associated therewith when a potential is applied to the electrode.

It is an object of this invention to reduce the undesirable effect of the electric field or ions entering a quadrupole mass filter. I

It is a further object of this invention to increase the It is a still further object of this invention to reduce the fringing field scattering of ions at both the entrance and exit ends of a quadrupole type mass filter by the use of resistive conducting electrodes having a thin conductive film thereon.

It is a still further object of this invention to improve the overall operationand performance of a quadrupole mass analyzer.

It is yet another object of this invention to improve the operation of a monopole mass analyzer.

The above and other objects and features of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings and claims which form a part of this specification.

FIG. 1 illustrates an ideal shape for electrodes used in a quadrupole mass spectrometer.

Invention FIG. 3,is a side view of electrodes that utilize the principles of this invention.

FIG. 4 is apartial cross-sectional view of an electrode having a conductive thin film thereon.

' FIG. Sis a partial cross-sectional view of an electrode having a conductive film thereon of varied thickness.

FIG. 6 is a block diagram of a monopole type mass analyzer which utilizes the principles of the invention. FIG. 7 is an end view of the monopole electrodes.

DETAILED DESCRIPTION OF THE DRAWINGS Prior Art FIG. 1 illustrates four electrodes A, A, B, B, of hyperboloidal shape arranged at a distance r,, from the z axis of an x, y, z coordinate system..The electrodes receive a potential which creates a time varying symmetrical electric field between them. The potential of the field is periodic in time, symmetrical with respect to the z-axis, and-depends quadratically on x and y:

f (t) is an arbitrary periodic function of time (t) and the field is assumed to be quasi-static, so that Laplaces equation A requires that the constants a and b should satisfy thecondition a b. The two electrodes A,A are electrically interconnected, and the electrodes B, B are electrically interconnected. A time-periodical voltage U =-U,, V coswt to electrode B whereby an electric field is created between the electrodes. This field is independent of z, its center of symmetry being the z axis.

FIG. 2 shows a schematic diagram of how voltage sources are coupled to the electrodes 1 of a quadrupole filter. Four elongated or cylindrical rod-shaped elec-'- trodes 1, which sufficiently approximate the field generated by hyperboloidal electrodes, are arranged to make a quadrupole mass filter. An electric generator 9 creates a high-frequency voltage V coswt which is applied to. the electrodes 1 through two capacitors 14. 'In this arrangement, the high frequency voltage is rectified and smoothed by a rectifier 10. The direct voltage so created is divided by a potentiometer l and coupled to the electrodes 1 .through inductors 13 whereby the ratio (u) of the direct voltage U to the alternating voltage V is substantially independent of alterations of the alternating voltage. As an alternative, a direct current voltage may be supplied to the electrodes l by an independent voltage source 11 and an electric two-way switch 11A.This electrode arrangement establishes an electric field between the electrodes whereby ions brought into such a field have equations of motion that are differential equations with periodical coefficients, the equations being characterized by having ranges of stable and unstable solutions. Thus, there exists two different kinds of ion paths (stable and unstable); either the ions perform oscillations around the center of symmetry of the field (z axis) because the amplitudes of the oscillations remain smaller than a certain maximum value (stable paths) or the amplitudesof the oscillations increase until they exceed a certain maximum value (unstable paths) whereby the ions impinge upon the field generating electrodes and are removed. For a more detailed discussion of quadrupole type mass filters see U.S. Pat. No. 2,939,952 to W. Paul et al.

Preferred Embodiments FIG. 3 illustrates a partial view of a quadrupole type .mass filter arrangement comprised of elongated electrodes l arranged in spaced relationship and held in fixed position by end plates 3. The.,volume between the end plates 3 and between the elongated electrodes 1 defines the filter region where ions of predetermined mass-to-charge ratios are separated from a group of ions entering the filter at one end.

The elongated electrodes 1 are preferably glass rods, having their surfaces treated so as to have a resistive conductor surface 4 having a sheet resistance in the range of 5 to 10 ohms per square as opposed to an untreated glass surface that has a sheet resistance. of about 10 ohms per square or greater. Deposited on the resistive surface 4 of the rod 1 is a thin conductive film 2 of metal having a sheet resistance of ohms per square or less. I

The conductive film 2 on the rod l may be deposited on the glass rod by various methods-However, the preferred techniques for depositing a conductive film 2 of controlled thickness on the rod 1 are sputtering, evaporative coating, or a combination of both. These methods are preferred because of the high degree of controllability over the thickness ofv the material deposited on the rod. For examples of sputtering and vaporizing techniques see U.S. Pat. No. 3,492,215 to L. A. Conant; No. 3,487,000 to D.-G. ,I-lajzak; and No.

3,451,917 to R. M. Moseson. Since the rod 1 is basically insulating material, e.g., glass, a radio frequency sputtering technique is one preferred technique for depositing a conductive coating 2 on the rod 1. The length L of the film 2 is chosen so that when an electric potential is applied to the rods 1, the electric field formed therebetween will assume a preferred shape. In this embodiment a dc potential is applied to the resistive conducting surface 4 at both ends of the rod 1 and the conducting film 2 at one end thereof. A resistor of resistors may be placed in the circuit to obtain various applied voltage ratios. I-Iowever, regardless of the length L of the conductive film 2, the film 2 does not make contact with the end plates 3. This assures that the potential applied to the conducting film 2 is not applied to the end plates 3. v

In this embodiment, the additional resistive surface coating 4, which has a different sheet resistance per square than the conductive film 2, is used to establish a predetermined electric field gradient at the end portion of the electrodes when a voltage is applied to the resistive and conducting surfaces. To obtain a resistive conducting surface, glass with a high lead content can be heated to a high temperature in a hydrogen atmosphere to obtain a surface having a sheet resistance less than 10* ohms per square. For alternate examples of other methods of applying resistive coatings, such as metal oxides on glass, see US. Pat. No. 2,9l5,730 to J. K. Davis; No. 3,069,294, to J. K. Davis; and No. 3,093,508 to E. W. Wartenberg.

FIG. 4 is a partial cross sectional view of an electrode 1 showing the resistive conductor surface 4 and the thin conductive film 2. Although a resistive film 4 is shown below the conductive film 2, an alternate arrangement not having a resistive film 4 below the conductive film is also acceptable. The thin conductive film 2 is a thin film of metal less than 25,000 Angstroms thick and for the purposes of this invention is generally in the range of 100 to 10,000 Angstroms. This results in a thin conductive film having a sheet resistance of 100 ohms per square or less. The resistive conducting surface 4 is about 5 ,000 Angstroms thick and has a sheet resistance of to 10" ohms per square and is obtained by any of the methods previously referenced.

FIG. 5 is a partial cross sectional view of an electrode 1 having a thin conductive (metal) film 2 thereon of varied thickness. The purpose of this is to obtain. a predetermined gradient of the electric field at one portion of the electrode.

FIG. 6 is a schematic block diagram of a monopole mass filter. An ion source 42 is connected to a currentsupply unit 21 which is preferably energized from a utility line. The ion accelerating voltage, for example about 30 volts, is then connected between the ion source and the angle structure comprised of two surface members 5 and 6. The ions are produced by electron collision in the ion source 42. The field electrode 1 is connected to a high frequency generator 23 and a direct voltage source 25 through a circuit containing a lead 15, a capacitor 22 and an inductance coil 24. The high frequency generator 23 and the direct-voltage source 25 are preferably energized by the same power source coupled to current supply 2l.A collector 47 is grounded through an input stage of an amplifier 26 whose output'circuit is connected to a recording instrument 27. In this filter, as in the previously described filters,the ions are introduced into the filter from a position which is generally along the z-axis and, therefore,

ions must pass the field at the ends of the electrodes (and the fringing field) when entering and leaving the mass filter. A more detailed description of a monopole mass filter may be found in US. Pat. No. 3,197,633 to U. VonZahn.

The field electrode 1 includes a thin metallic film 2, deposited by vapor or sputtering deposition techniques. The remaining surface 4 of the field electrode at the ends is preferably a resistive conductor. Therefore, the field electrode 1 has different electric resistances per square along portions of its surface that will affect the shape of the electric field associated with the field electrode 1.

FIG. 7 is a cross-sectional view taken along lines Vl-Vl of FIG. 5. This view shows that the conductive coating 2 on the glass field electrode 1 is adjacent surface members 5 and 6.

While a preferred embodiment of the invention has been disclosed, it will be apparent to those skilled in the art that changes may be made to the invention as set forth in the appended claims, and, in some cases, certain features of the invention may be used to advantage without corresponding use of other features. For example, although the invention has been illustrated as useful in a quadrupole type mass filter, it is also useful in duopole, monopole, or similar type filters. Accordingly, it is intended that the illustrative and descriptive materials herein be used to illustrate the principles of the invention and not to limit the scope thereof.

Having described the invention what is claimed is:

1. A multipole mass filter for separating charged particles according to their mass-to-charge ratio, which comprises:

a plurality of rods comprised of an insulating material each having a first film thereon having a resistivity between 10 and l0 ohms per square;

means for mounting said rods symmetrically in a substantially parallel relationship about a central axis;

a second film comprised of metal disposed on each of said rods at least along the surface thereof that faces said central axis, said metal film separated from said mounting means by said first film, said metal film being substantially uniform in thickness and having a resistivity of less than ohms per square;

means for introducing charged particles into said filter, said means including a source of ions at one end of said rods;

means for detecting charged particles leaving said filter, said detecting means located at the other end of said rod;

a source of a-c voltage;

means for applying said a-c voltage to the metal film on said rods to produce an alternating multipole electric field between said rods;

a source of d-c voltages of variable magnitudes;

means for applying selected d-c voltages from said d- 0 source to said metal film on said rods to produce different static multipole electric field components between said rods; and

means for adjusting the ratio of the static electric field to the alternating electric field to select charged particles of a predetermined mass-tocharge ratio for transmission through the filter to the detector means and to'deflect charged particles of other mass-to-charge ratios away from the central axis.

2. A multipole mass filter for separating charged particles as recited in claim 1 wherein the metal film disposed on each of said rods is less than 25,000 angstroms thick.

3. A multipole mass filter for separating charged particles according to claim 2 wherein the first film on the rods is formed by a coating less than 5,000 angstroms thick.

4. The mass filter as recited in claim 3 wherein said rods are comprised of glass.

5. The mass filter as recited in claim 4 wherein each of said rods has an additional metal film at one end portion of said second film, said additional film having a thickness which is less than the thickness of said second metal film.

6. The mass filter as recited in claim 3 wherein each of said rods has an additional metal film at one end portion of said second film, said additional film having a thickness which is less than the thickness of said second metal film.

7. The mass filter as recited in claim 2 wherein said rods are comprised of glass.

8. The mass filter as recited in claim 7 wherein each of said rods has an additional metal film at one end portion of said second film, said additionalv film having a thickness whichis less than the thickness of said second metal film.

9. The mass filter as recited in claim 2 wherein each of said rods has an additional metal film at one end por-,

tion of said second film, said additional film having a thickness which is less than the thickness of said second said second metal film. I

13. The mass filter as recited in claim 10 wherein each of said rods has an additional metal film at one end portion of said second film, said additional film having a thickness which is' less than the thickness of said second metal film.

14. The mass filter as recited in claim 1 wherein each of said rods has an additional metal film at one end portion of said second film, said additional film having a thickness which is less than the thickness of said second metal film.

15; The mass filter as recited in claim 1 wherein said rods are comprised of glass.

16. The mass filter as recited in claim 15 wherein each of said rods has an-additional metal film at one end portion of said second film, said additional film having a thickness which is less than the thickness of said second metal film. 

1. A multipole mass filter for separating charged particles according to their mass-to-charge ratio, which comprises: a plurality of rods comprised of an insulating material each having a first film thereon having a resistivity between 105 and 1010 ohms per square; means for mounting said rods symmetrically in a substantially parallel relationship about a central axis; a second film comprised of metal disposed on each of said rods at least along the surface thereof that faces said central axis, said metal film separated from said mounting means by said first film, said metal film being substantially uniform in thickness and having a resistivity of less than 100 ohms per square; means for introducing charged particles into said filter, said means including a source of ions at one end of said rods; means for detecting charged particles leaving said filter, said detecting means located at the other end of said rod; a source of a-c voltage; means for applying said a-c voltage to the metal film on said rods to produce an alternating multipole electric field between said rods; a source of d-c voltages of variable magnitudes; means for applying selected d-c voltages from said d-c source to said metal film on said rods to produce different static multipole electric field components between said rods; and means for adjusting the ratio of the static electric field to the alternating electric field to select charged particles of a predetermined mass-to-charge ratio for transmission through the filter to the detector means and to deflect charged particles of other mass-to-charge ratios away from the central axis.
 2. A multipole mass filter for separating charged particles as recited in claim 1 wherein the metal film disposed on each of said rods is less than 25,000 angstroms thick.
 3. A multipole mass filter for separating charged particles according to claim 2 wherein the first film on the rods is formed by a coating less than 5,000 angstroms thick.
 4. The mass filter as recited in claim 3 wherein said rods are comprised of glass.
 5. The mass filter as recited in claim 4 wherein each of said rods has an additional metal film at one end portion of said second film, said additional film having a thickness which is less than the thickness of said second metal film.
 6. The mass filter as recited in claim 3 wherein each of said rods has an additional metal film at one end portion of said second film, said additional film having a thickness which is less than the thickness of said second metal film.
 7. The mass filter as recited in claim 2 wherein said rods are comprised of glass.
 8. The mass filter as recited in claim 7 wherein each of said rods has an additional metal film at one end portion of said second film, said additional film having a thickness which is less than the thickness of said second metal film.
 9. The mass filter as recited in claim 2 wherein each of said rods has an additional metal film at one end portion of said second film, said additional film having a thickness which is less than the thickness of said second metal film.
 10. A multipole mass filter for separating charged particles according to claim 1 wherein the first film on the rods is formed by a coating less than 5,000 angstroms thick.
 11. The mass filter as recited in claim 10 wherein said rods are comprised of glass.
 12. The mass filter as recited in claim 11 wherein each of said rods has an additional metal film at one end portion of said second film, said additional film having a thickness which is less than the thickness of said second metal film.
 13. The mass filter as recited in claim 10 wherein each of said rods has an additional metal film at one end portion of said second film, said additional film having a thickness which is less than the thickness of said second metal film.
 14. The mass filter as recited in claim 1 wherein each of said rods has an additional metal film at one end portion of said second film, said additional film having a thickness which is less than the thickness of said second metal film.
 15. The mass filter as recited in claim 1 wherein said rods are comprised of glass.
 16. The mass filter as recited in claim 15 wherein each of said rods has an additional metal film at one end portion of said second film, said additional film having a thickness which is less than the thickness of said second metal film. 